For Tax Forms and Booklets
Your one-stop (free) shop for every tax form and booklet you could ever need. If your company has even one employee, make sure to download Circular E; it's basically an employer's guide to everything about federal payroll taxes. On this comprehensive and easy-to-navigate site, you'll also be able to easily find estimated tax payment calculators and coupons (both for individuals and C corporations).
This site can help steer you in the right direction when preparing your taxes and answer most of your questions along the way, including what forms you'll need to fill out and where to find them.